fix @differentiable function in .swiftinterface (#25059)

While messing with .swiftinterface files, I discovered that `@differentiable` functions don't get printed correctly in .swiftinterface files.

The test in `test/AutoDiff/differentiable_attr_parseable_interface.swift` demonstrates what this fixes.

The extra test in `test/Serialization/differentiable_attr.swift` was already passing before this change, but I noticed that that file didn't have a `@differentiable` function as a param, so I thought it would be good it to add to that file too.

Author: marcrasi

lib/AST/TypeRepr.cpp

@@ -299,6 +299,10 @@ void AttributedTypeRepr::printAttrs(ASTPrinter &Printer,
   if (hasAttr(TAK_escaping))
     Printer.printSimpleAttr("@escaping") << " ";
 
+  // SWIFT_ENABLE_TENSORFLOW
+  if (hasAttr(TAK_differentiable))
+    Printer.printSimpleAttr("@differentiable") << " ";
+
   if (hasAttr(TAK_thin))
     Printer.printSimpleAttr("@thin") << " ";
   if (hasAttr(TAK_thick))

test/AutoDiff/differentiable_attr_parseable_interface.swift

@@ -0,0 +1,6 @@
+// SWIFT_ENABLE_TENSORFLOW
+// RUN: %target-swift-frontend -typecheck -emit-parseable-module-interface-path %t.swiftinterface -enable-library-evolution %s
+// RUN: %FileCheck %s < %t.swiftinterface
+
+// CHECK: func testDifferentiableParam(f: @differentiable (Float) -> Float)
+public func testDifferentiableParam(f: @differentiable (Float) -> Float) {}

test/Serialization/differentiable_attr.swift

@@ -97,3 +97,6 @@ extension P where Self : Differentiable, Self == Self.TangentVector {
     return (self, { v in v })
   }
 }
+
+// CHECK: func testDifferentiableParam(f: @differentiable (Float) -> Float)
+func testDifferentiableParam(f: @differentiable (Float) -> Float) {}